So i see a lot of people have purple stems and wonder how the hell it happens. Well for those who ask the answer is all too often 'GENETICS' or temps. I see it as a sign of overwatering, seedlings need very little water indeed, you really would be suprised. If the roots stay wet for too long then you get purpling of the stem from phosphorous deficiency/lockout. I grow the same seeds from the same strain and same mother, i use peat cubes but only till the seed germinates at which point the peat cube holds too much water for my liking, its like a sponge not good for drainage. So i prick the seedling out of the peat cube and replant it in my seedling soil that is amended with perlite or vermiculite. The pics show what a healthy seedling stem should look like, pics one and two notice no purple stem or leaf veins or leaf! This is the only colour you want your seedling stem to be, unless you are growing a purple strain of which very few do or have. It is not genetics 9 out of ten times but simply overwatering. Myth solved but open to discussion!? Sorry last pic is my tumbling tom cherry tomato plant.